Oil filled pressure transducers are well known. The prior art shows such devices that have been used with oil, where oil or fluid is employed as a force-transmitting medium. In such devices, the pressure is applied to a transducer diaphragm, and typically these devices employ a metal diaphragm as a force collector. Such oil filled transducers are known in the art. For example, see U.S. Pat. No. 4,406,993 entitled, “Oil Filled Pressure Transducers” issued on Sep. 27, 1983 to Anthony D. Kurtz, the inventor herein, and assigned to Kulite Semiconductor Products, Inc., the assignee herein. See also U.S. Pat. No. 6,330,829 entitled, “Oil Filled Pressure Transducer” issued Dec. 18, 2001 to A. D. Kurtz et al., the inventor herein, and assigned to Kulite Semiconductor Products, Inc., the assignee herein.
It is well known that the pressure caused by a deflection (δ) of a clamped edge diaphragm of thickness (t) and radius (a) is given by:
  P  =                    16        ⁢        δ        ⁢                                  ⁢                  Em          2                    +      3              3      ⁢                        a          4                ⁡                  (                                    m              2                        -            1                    )                    Where m=reciprocal of Poisson's Ration
P=pressure
E=Young Modulus
In an oil-filled pressure transducer, the sensor element is isolated from the pressure media by means of a thin metal diaphragm and the pressure is transmitted to the sensor by means of a volume of oil mainly in the form of a film between the metal diaphragm and the sensor. This is, of course, indicated in the above-noted patents. In any event, when the temperature increases, the oil expands and pushes against the metal diaphragm thus exerting a backpressure against the sensor leading to an error signal. This problem is known and is addressed and explained in U.S. Pat. No. 5,999,082 entitled, “Compensated Oil Filled Pressure Transducer” issued on Dec. 7, 1999 to A. D. Kurtz et al., the inventor herein and assigned to the assignee herein. In that patent, the object was to reduce errors at very low pressure caused by the oil exerting a tension on the deflecting portion of the diaphragm. A first attempt to make a minimum oil volume transducer using a leadless sensor structure was shown in U.S. Pat. No. 6,591,686 entitled, “Oil Filled Pressure Transducer”.
The prior art describes an oil filled pressure transducer which utilizes a leadless sensor which is secured to a header comprising a glass pre-form and a header shell. The glass pre-form contains holes which accept header pins and another aperture or hole which accepts the oil fill tube. The diameter of the sensor was chosen to be almost as large as the inner diameter of the shell. In this manner, there was a small cut out over the portion of the sensor that would otherwise cover the oil fill tube. The sensor was mounted to the header using glass bonds. There was a very small space between the outer diameter of the sensor and the inner diameter header housing, which was filled with glass used to mount the sensor. Since there were no ball bonds or gold wires in the area between the surface of the sensor and the diaphragm the distance between the sensor and metal diaphragm was drastically reduced, thereby substantially reducing the backpressure problem. However, there are still a number of remaining problems with this construction. Making the sensor diaphragm chip the same size as the inner hole is not appropriate. It is difficult to line up the location of the header pins with respect to the apertures in the sensing diaphragm. Using this approach, it is very difficult to maintain a close separation between the metal diaphragm and the underlying sensor over a large diameter. It is very difficult to orient the sensing diaphragm with respect to the pins, thus making the mounting exceedingly difficult. It is an object of this invention to reduce the oil volume while enabling easier sensor assembly. It is another object of the present invention to make the effective diameter of the isolation diaphragm much larger to reduce the back pressure.
It is an object of the present invention to reduce the oil volume to a minimum volume expansion and thus reduce the error causing backpressure.